Thanks Kato that was what I had been wondering.
And isn't it some kind of weakness?? because no one can guarantee that Patriot itself could destroy everything that is flying against it.
It really depends on a given force's doctrine, and whether they have the platforms available to provide the system response called for by doctrine.
AFAIK Patriot SAM batteries at this point are principally intended to provide BMD, despite being originally intended to provide air defence against bombers IIRC.
From the standpoint of a BMD system, a Patriot battery would be positioned to protect a relatively immobile theatre-level strategic target. This might be something like a major base, HQ, supply dump, population centre etc. The sort of target which an opposing force might be inclined to attempt to strike at via ballistic missiles. Such a target would also most likely be behind the front lines.
With this in mind, that the Patriot battery is positioned behind the front lines, it would not come under air attack unless the opposing force had aircraft carry out a strike upon the battery. In order to do that, the hostile aircraft would need to penetrate into airspace that is monitored by US C5ISR systems and would have patrolling fighters. This is also assuming that the US has not already eliminated the aircraft the hostile force would have to employ.
This in turn takes one back to how the US conducts military campaigns in the modern era... One of the first things which is done is a rollback of an enemies IADS early on. A part of which is the disabling, if not outright destruction of the enemies air force.
Therefore, in order for a Patriot battery to come under air attack, it would have to be a situation where the US has ground troops deployed sufficiently to have a Patriot, yet not have control of the air. This could possible occur early in a campgian if the US were to take aggressive action against another nation with significant aircraft and IADS assets. Or as an alternative, if nation like I described were to launch an attack (particularly a surprise one) upon the US, US assets and forces... IMO neither of these scenarios are particularly likely.
-Cheers